1. Field of the Invention
This invention relates to a liquid fertilizer composition which is capable of supplying sulfur, nitrogen and micronutrient metals for plant nutrition. More particularly, it relates to an aqueous solution of ammonium thiosulfate, ammonia and at least one metal compound selected from the group consisting of the oxides and salts of copper, zinc and manganese.
2. Description of the Prior Art
Plants require a variety of materials for adequate nutrition. The principal materials are carbon dioxide and water, which provide the elements of carbon, hydrogen and oxygen, and which are usually available in adequate amounts from both soil and atmosphere. The supply of carbon dioxide and water is continually replenished by natural phenomena. All the other essential nutrients are normally available from the soil, but they are not replenished by nature after the plant takes them up into its foilage and fruit. When the plant is removed from the soil for consumption, in contrast to the natural cycle of return to the soil, the soil nutrient supply is reduced, and the application of fertilizer becomes necessary in time.
Thirteen elements, which are normally drawn from the soil, are currently known to be essential to plant nutrition. These elements consist of nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, copper, zinc, boron, molybdenum and chlorine. Of these thirteen elements, nitrogen, phosphorus and potassium are needed by plants in relatively large quantities and are therefore called macronutrients. Calcium, magnesium, and sulfur are generally required in lesser but still substantial amounts and are classified as secondary nutrients. The remaining members of the group are known as micronutrients since they are required in very small amounts for plant growth.
Plants obtain sulfur from both the soil and the air. In the soil, sulfur is ordinarily present either in the form of sulfur containing inorganic minerals or as organic sulfur compounds in the soil organic matter or humus. In the air, sulfur is primarily present in the form of sulfur dioxide which can be directly absorbed by the leaves of plants. In addition, sulfur dioxide from the air is also carried into the soil by rain water where it may be absorbed by plants.
The discharge of sulfur oxides into the air by some large industrial centers is sufficiently large to ensure that a sulfur deficiency does not develop in nearby agricultural land. This, however, is not the case for agricultural land which is remote from such sources of atmospheric sulfur oxides. In view of limitations on the natural supply of sulfur from both air and soil, a sulfur deficiency is not uncommon in agricultural soils. Such a deficiency frequently occurs in well-leached soils containing little organic matter which are located in areas far from sources of sulfur dioxide atmospheric pollution. Crops which appear to be particularly sensitive to a sulfur deficiency include corn, sugar cane, wheat, sugar beets, and legumes such as alfalfa and peanuts.
One of the most satisfactory sources of sulfur for plant nutrition is ammonium thiosulfate, which simultaneously serves as a source of nitrogen. In addition, ammonium thiosulfate contains more sulfur per unit weight than ammonium sulfate which is a widely used fertilizer material. Ammonium thiosulfate is conveniently handled as a concentrated aqueous solution and is compatible with many liquid fertilizer solutions. It is compatible with neutral phosphates, aqueous ammonia, nitrogen solutions containing ammonium nitrate, urea solutions, and most nitrogen, nitrogen-phosphorus, or complete fertilizer solutions.
Although micronutrient metals can be separately applied to agricultural soil, it is generally more convenient to incorporate these micronutrients into conventional mixed fertilizers which are used to supply nutrients such as nitrogen, phosphorus, potassium and sulfur. The satisfactory incorporation of micronutrient metals into a solid mixed fertilizer is a major problem because of difficulties such as the uniform incorporation of the small amounts of material and the prevention of segregation from mixing. These specific difficulties can be avoided by incorporating the micronutrient metals into a liquid fertilizer. Unfortunately, many of the metal compounds conventionally employed as a source of micronutrient metals are of limited solubility in many conventional liquid fertilizer compositions which are used to supply nutrients such as nitrogen, phosphorus, potassium and sulfur.
In view of the desirable characteristics of aqueous ammonium thiosulfate solutions as a source of sulfur, there has been a need for a satisfactory method of incorporating micronutrient metals into such solutions. This has been a substantial problem in the past since micronutrient metals such as copper, zinc and manganese form thiosulfates which precipitate from solution at the concentrations that are required for efficient transportation and storage.
U.S. Pat. Nos. 3,854,923; 3,909,229; and 3,997,317 are directed to a micronutrient composition consisting of a solution in anhydrous or aqueous ammonia of a zinc alkanoate which contains from 1 to 6 carbon atoms. These patents teach that the zinc salts of low molecular weight alkanoic acids are desirable sources of zinc for use in combination with ammonia as a consequence of their relatively high solubility in aqueous or anhydrous ammonia. These patents do not, however, suggest for any purpose the preparation of a fertilizer composition by mixing water, ammonium thiosulfate, ammonia and at least one metal compound selected from the group consisting of the oxides and salts of copper, zinc and manganese.
Similarly, U.S. Pat. No. 4,007,029 discloses the preparation and use of a liquid fertilizer composition which comprises anhydrous ammonia, at least one trace element selected from the group consisting of boron, copper, magnesium, manganese and molybdenum, and wherein a compound such as ammonium acetate, magnesium acetate or sodium acetate can be used to solubilize the trace element. This patent fails to suggest, for any purpose, the combination of water, ammonium thiosulfate, ammonia and at least one metal compound selected from the group consisting of the oxides and salts of copper, zinc and manganese.
U.S. Pat. No. 2,976,138 is directed to the discovery that a stable aqueous solution of micronutrient metals can be prepared by solubilizing the metals with ammonia and ammonium sulfate. It is taught that the ammonium sulfate assists in the formation of complex ammoniacal sulfate salts of the metals which afford a stable aqueous solution at relatively high metal concentrations. This stabilized micronutrient solution has not been entirely satisfactory, however, as a consequence of its relatively low sulfur content and the tendency of manganese to oxidize to manganic oxides and precipitate from solution. The disclosure contains no suggestion that an aqueous solution prepared by mixing water, ammonium thiosulfate, ammonia and at least one metal compound selected from the group consisting of the oxides and salts of copper, zinc and manganese would be free of such deficiencies. Indeed, the disclosure fails to suggest the possibility that such a solution could be prepared.
In addition, U.S. Pat. No. 2,957,762 discloses the use of a water-soluble stable ammine complex salt of a metal selected from Groups Ib, IIb, VIa and VIII of the Periodic Table to inhibit the corrosive activity toward ferrous metal of aqueous ammonia solutions of ammonium salts of strong mineral acids. The ammine complex salt disclosed by this patent is used at a concentration of between about 0.0001 and about 0.01 part by weight per part of the combined weight of the ammonia and ammonium salt. This patent fails to suggest the possibility of preparing a solution containing a high metal content by mixing water, ammonium thiosulfate, ammonia and at least one metal compound selected from the group consisting of the oxides and salts of copper, zinc and manganese. The patent is directed solely to the ammonium salts of strong mineral acids whereas the ammonium thiosulfate of this invention is not such a salt. In addition, the patent fails to suggest the possibility of attaining the high metal concentrations which are made possible by the present invention.